UNITED NATIONS — The ailing father of a former U.S. Marine imprisoned in Iran is pleading for the release of his son in a letter delivered Wednesday to Iranian President Hasan Rouhani, in New York to attend a meeting of world leaders.

Ali Hekmati is asking Rouhani to order the release of his son, Amir, who was arrested in Iran in 2011. The family says Amir Hekmati, who has dual U.S. and Iranian citizenship, was visiting his grandmother. Iran accused him of being a CIA spy and convicted him.

“I long more than ever to see Amir’s face. I am now very sick with a brain tumor,” Ali Hekmati wrote in the letter, which was delivered to Rouhani’s delegation by an Islamic religious official from Michigan who has a personal relationship with the Iranian president, according to a representative for the family.

“I ask that you let me see him again, one more time, and so that he may lead our family when I am gone,” Hekmati wrote in his letter. “Amir is a good man. An honorable man. He is not a spy, I can assure you of that.”

A spokesman at the Iranian Mission to the U.N. did not immediately respond to a request for comment.

The family has cause for hope: Rouhani’s moderate tone could signal a warming of relations with the United States. Iran freed dozens of political prisoners in the last week, just as Rouhani was headed to New York for the annual meeting of the United Nations General Assembly.

Earlier this month, Hekmati wrote a letter to Kerry describing his “miserable prison conditions” and his belief that Tehran wanted to use him in a possible prisoner exchange.

Hekmati’s letter was smuggled out of prison. His sister authenticated the handwriting. The State Department said it was trying to determine Hekmati’s condition through Swiss diplomats in Tehran. The Swiss represent U.S. interests in Iran because no U.S. officials are based there. The two countries severed diplomatic relations in 1979.

Two other American citizens are believed to be detained in Iran: Retired FBI agent Robert Levinson and Christian pastor Saeed Abedini.